Taxonomy & naming
Girardinus microdactylus was described by the Cuban-American ichthyologist Luis René Rivas in 1944, as part of his broader work on the freshwater fishes of Cuba and the wider Caribbean. Eschmeyer's Catalog of Fishes treats Girardinus microdactylus Rivas, 1944 as the valid name, placing the species in the genus Girardinus within the subfamily Poeciliinae of the family Poeciliidae — the New World livebearers that also contain the guppies, mollies, platies and swordtails.
The genus Girardinus is a small Cuban-centred radiation; its best-known member is Girardinus metallicus, the metallic topminnow, which is the congener most often seen in the aquarium trade. Girardinus microdactylus is far more obscure, an endemic of Cuba that rarely leaves the island either physically or in the literature. The genus name honours Charles Frédéric Girard (1822–1895), the French-born ichthyologist and herpetologist who worked as a pupil and assistant of Louis Agassiz, and the specific epithet microdactylus — from the Greek for "small finger" — gives the fish its English name, the smallfinger topminnow.
Morphology
Girardinus microdactylus is a small poeciliid. FishBase records a maximum of about 3 in standard length in males and unsexed fish and roughly 2.5 in standard length in females, making it a modestly sized livebearer comparable in scale to its better-known relatives. As with other members of the subfamily Poeciliinae, the sexes differ: males carry the gonopodium, an intromittent organ formed from modified anal-fin rays, while females are deeper-bodied and lack it.
Detailed published descriptions of live colouration and fine meristics for this species are sparse, and the cached sources do not provide a full morphological account. In keeping with the editorial rule of saying less rather than guessing, only the well-supported figures are stated here: a small, benthopelagic body plan typical of Girardinus, with the genus-wide poeciliine traits of internal fertilisation and male gonopodium development. Aquarists encountering the fish should rely on collection data and the authority's original description rather than on assumed colour patterns.
Habitat
Girardinus microdactylus is endemic to Cuba, where it lives in fresh water as a benthopelagic, non-migratory fish. FishBase records it from highland streams, where it is found in pools and along the banks and occasionally ventures into swift water. It swims at midwater and along the bottom, and comes to or near the surface only occasionally — a habit consistent with the "topminnow" name applied across the genus.
The species is reported to be sensitive to high chlorine levels and to warm, turbid, poorly oxygenated, stagnant water, which points to a fish adapted to the clean, moving waters of upland Cuban streams rather than to degraded lowland pools. Recorded water conditions are tropical and moderately hard to hard: FishBase gives a temperature range of about 72–79 °F and a hardness (dH) range of roughly 10–25, indicating neutral-to-alkaline, mineral-rich water. As a Cuban endemic its entire natural range is confined to the island's freshwater systems.
Feeding
In the wild Girardinus microdactylus is a grazer and micro-omnivore. FishBase reports a diet of diatoms, algae, detritus and insect larvae, and assigns the species a low trophic level of about 2.3 — placing it firmly among the herbivore-detritivores that lean heavily on plant and algal matter while supplementing with small invertebrates. This matches its habit of working the stream bottom and banks, where biofilm, algae and settled organic material are abundant.
No species-specific captive feeding regime is documented in the cached sources. By analogy with its diet and with the husbandry of congeners such as Girardinus metallicus, a varied diet built around good-quality flake or micro-pellets with a substantial share of vegetable and algal matter, plus occasional small live or frozen invertebrates, would suit it. Because the species is a near-constant grazer with a low trophic level, frequent small feedings rather than single large meals are appropriate, and a steady supply of algae or green foods is likely to keep it in good condition.
Mating
Like all members of the subfamily Poeciliinae, Girardinus microdactylus reproduces by internal fertilisation rather than by laying eggs. The male transfers sperm to the female using the gonopodium, a slender intromittent organ built from modified rays of the anal fin — the defining structure of the live-bearing poeciliids and the morphological hallmark that separates mature males from females.
Species-specific courtship behaviour for Girardinus microdactylus is not documented in the cached sources. In the genus more broadly, males are persistent and use the gonopodium to inseminate females, after which females can carry and nourish developing young internally. Data on whether this species shows superfetation — the poeciliid trait in which a female carries broods at more than one stage of development simultaneously — are not available here, and so no claim is made either way; the reproductive account is therefore limited to the well-established genus- and family-level pattern of internal fertilisation via the gonopodium.
Breeding
Girardinus microdactylus is viviparous: the female retains the fertilised eggs internally and gives birth to free-swimming young, the universal reproductive mode of the Poeciliinae. After insemination via the gonopodium, embryos develop inside the female and are released as fully formed fry able to swim and feed at once.
The cached sources give no species-specific brood size, gestation length or breeding cadence for Girardinus microdactylus — this is an uncommon Cuban endemic that is rarely kept, and reliable figures simply are not available. Rather than invent numbers, the data here are best treated as sparse. What can be said with confidence is the mode (live-bearing) and the general livebearer pattern that the newborn fry will need cover, since adult poeciliids commonly predate their own young; dense planting or floating cover would be the standard precaution for anyone attempting to raise broods. Precise fecundity should be drawn from direct observation or future literature rather than assumed.
In the aquarium
Girardinus microdactylus is uncommon in aquaria; the genus is represented in the hobby chiefly by Girardinus metallicus, and no dedicated specialist husbandry page for the smallfinger topminnow was located in the cached research. Care guidance here is therefore inferred from its recorded wild requirements rather than from established aquarium practice, and should be treated accordingly.
The species' habitat data point to clean, well-oxygenated, moving water that is moderately hard to hard and neutral-to-alkaline, at tropical temperatures of roughly 72–79 °F. Its documented sensitivity to high chlorine and to warm, turbid, stagnant, poorly oxygenated conditions is the key husbandry caution: this is a fish for a well-filtered, well-aerated, stable aquarium with good water quality rather than a still, crowded tank. As a small, peaceful, grazing livebearer it would suit a quiet community of similarly sized fishes, with hard, alkaline water and a steady supply of algae and small foods.
Conservation
Girardinus microdactylus is assessed as Least Concern on the IUCN Red List, in an assessment dated 1 May 2020. Despite being a single-island endemic, it is not currently considered threatened — in contrast to a number of other narrow-range island and continental livebearers, particularly the Mexican goodeids, that face serious decline.
As a Cuban endemic, however, the species' fortunes are tied entirely to the freshwater systems of one island, and the same FishBase habitat notes that describe its biology also flag its vulnerabilities: a sensitivity to chlorine and to warm, turbid, oxygen-poor, stagnant water means that pollution, abstraction and the degradation of highland streams are the kinds of pressures most likely to matter for it. The Least Concern listing reflects current status rather than a guarantee, and ongoing monitoring of Cuba's endemic freshwater fauna remains the appropriate conservation posture.